Question about UVB tube lights

I have been using a reptiglo 5.0, on Sully's small cage. We are almost done with the build of his big cage and I am wondering if a 5.0 bulb is going to suffice?

I have seen 2.0, 5.0, 8.0, and 10.0.

Should I use a combination, stick with the 5.0, or get something higher since the cage is much bigger?

One idea is to use a UVB 10 and add a piece of screen between the tube and the top screen of the enclosure. After about half way through the bulbs suggested life you can remove the extra screen and it would be like having a "new" bulb again. Of course, if you have a light meter you would be able to have a more scientific answer .